If you're anything like me, you probably load and play with virtualized evaluation editions of Windows Server all the time and, every so often, you let one of your virtual installations lag a little and you start getting the black screen indicating that you're not using a valid copy of Windows. Different evaluation editions of Windows Server 2008/R2 come with different evaluation periods ranging anywhere from 60 to 120 days. Using a very simple command line tool, you can extend – rearm – your evaluation period multiple times to receive a total of up to 240 days of evaluation after which point it's time to create a new virtual machine if you intend to continue your testing.
The command: slmgr.vbs -rearm
Slmgr is the command line software license management tool built into Windows. The rearm parameter resets Windows' evaluation clock back to zero. This trick isn't limited to just server editions of Windows, either. It also works with Windows Vista and Windows 7.